1. Field of the Invention:
The present invention generally relates to a bipolar transistor, and particularly concerns an improvement in manufacturing of bipolar transistor having low base resistance and ideal graft-base configuration in the bipolar transistor.
2. Description of the Prior Art:
In order to achieve low noise and high frequency operation of bipolar transistors, as one measure of improvement, lowering of base spreading resistance has been proposed. One prior art example to achieve this is a graft-base configuration, wherein active base region is formed with low impurity concentration in order to improve carrier injection efficiency and a base contact region connecting thereto is formed with high impurity concentration, thereby to lower the base spreading resistance.
FIG. 1 shows a sectional configuration of the prior art bipolar transistor having the graft-base configuration, wherein on an n-type silicon substrate 1 of a high impurity concentration an n-type epitaxial layer 2 of a low impurity concentration is formed, and in the n-type epitaxial layer 2 p-type base contact region 3, 3 of a high impurity concentration and a p-type active base region 4 of a low impurity concentration are formed in sequence, and further, in the p-type active base region 4 of low impurity concentration an n-type emitter region 5 is formed, and finally, electrodes 6, 6" and 6' are formed on the base contact regions 3, 3 and on the emitter region 5 through openings of a silicon oxide film 7.
In the above-mentioned prior art method for manufacturing of the bipolar transistor, however, since impurity concentrations of the base contact region 3, 3 and the active base region 4 are different each other, formings of these regions 3, 3 and 4 require respective processes which require long time, many workers and much materials. Furthermore, on account of using mask resistrations in the above-mentioned two processes, gaps A between the periphery of the emitter region 5 and the periphery of the base contact region 3 becomes 2 .mu.m, or larger. Such size of the gap is not sufficiently small for enabling lowering of base resistance, and the base spreading resistance cannot be sufficiently lowered.